Tivo releases what they see as either not a competitive advantage or a burden to maintain. Just like Nvidia releases the shim but nothing relating the how the hardware works - it's a business they aren't here to plant daisies in the FOSS fields there here to make a bottom line.
At least some router manufacturers have also released source code as required by the GPL. For example, see D-Link http://tsd.dlink.com.tw/GPL.asp and Cisco http://homesupport.cisco.com/en-us/gplcodecenter .